LaTeX beamer fixed vertical alignment

I believe this is a better solution, which makes sure that the text, under the picture, will be kept in place.

\begin{frame}{Test}
\begin{center}
    \includegraphics[width=0.8\linewidth]{bird.jpg}
  \end{center}

\begin{center}
    \begin{overprint}
        \onslide<1>
            Text for the first slide.\\
            Maybe few lines\\
        long...
        \onslide<2>
            Replacement on the second slide.\\
        This time only two lines.
    \end{overprint}
\end{center}
\end{frame}

If the text is below the image, this shouldn't be to much of a problem.

\begin{frame}
  \begin{center}
    \includegraphics{foo.pdf}
  \end{center}

  \uncover<+>{text on first slide}%
  \uncover<+>{text on second slide}%
  \uncover<+>{text on third slide}

\end{frame}

What's more challenging is a situation where the static material is below the dynamic material. Then you have to put the dynamic material in a box that is the same size on each slide. The most common ways to do that are with the overlay and overprint environments (see the beamer manual).

You can use [t] specifier for the frame to be top-aligned, then the height of the contents below the picutre does not matter:

\documentclass{beamer}

\begin{document}

\begin{frame}[t]\frametitle{My Frame}

\begin{center}
 \rule{3cm}{3cm} % substitute for a picture
\end{center}

\begin{itemize}
\only<1>{
\item First point on first slide
\item Second point on first slide
}
\only<2>{
\item Only one point on second slide
}
\end{itemize}

\end{frame}

\end{document}